The present invention relates to a novel hydrokinetic (tidal or river or canal) energy conversion system (HKECS), which exploits the use of excess energy in ocean tides or river/canal streams to generate electricity or to power mechanical loads such as water pumping.
Clean, renewable energy sources, such as solar, wind, ocean wave and tides or river streams have become particularly relevant and the subject of growing research and development as fossil fuel alternatives. Increased awareness of global climate change due to harmful greenhouse gas emissions, harmful hazardous wastes from coal and nuclear energy, as well as the desire to move away from dependency on the depleting reserves of fossil fuels makes clean renewable energy sources economically and environmentally attractive, if not imperative.
However, some of these renewable energy resources are not globally available with equal or dependable energy densities. Different geographic locations and changing climate conditions make clean energy sources inconsistent or unpredictable. Moreover, their energy densities vary substantially. For example, solar energy is about 0.15-1 kW/m2 with the higher value near the equator; wind energy is about 0.2-1.0 kW/m2; ocean waves are about 10-50 kW/m; and tidal flows can provide from 0.5-20 kW/m2 for an annual average water stream velocity range of 1.0 to 3.5 m/s).
Tidal and river stream energy is more predictable than wind energy or solar power. Tides are driven by predictable and regular gravitational forces between the sun, moon and earth, whereas wind and solar power are governed by solar radiation and the interaction of atmosphere, ocean, topology and earth rotation, which often result in heterogeneous and unpredictable distribution. Natural river systems and man made canal systems operate on the gradient between the precipitation site and the ocean, which normally exceeds the required value to maintain the nominal flow regime and results in soil erosion. It is this excess kinetic energy that can be gainfully employed with extra side benefits, such as soil retention or recovery. For example, it is estimated that India receives about 400 million hectare-m rainfall annually, and if this occurs at an average altitude of 500 m then it possesses 2×1019 J or 5.5×109 MWhr of energy, which is five times the total current annual electricity generation in India.
Tidal energy varies with combined solar-lunar cycles in a cyclic fashion with four cycles per day with predictable phase shifts of about 50 minutes. The magnitude of tidal energy depends upon the strength of the tide, which is determined by the changing positions of the moon and sun, the effects of the earth's rotation, and the local shape of the sea floor and coastlines. In particular, it is known that a current flowing against the swell increases the wave height, whereas the wave height is attenuated when both phenomena are in the same direction. Seasonal variations and geographic tidal location also need to be taken into account. Although tidal energy on a given site can be predicted if the local tidal streams are known, the tidal basin shape and other factors influence tidal energy and can be important design considerations. The attractive zones for tidal energy systems are in areas with fast currents where natural flows are concentrated in restricted coastal configurations, like at the entrances to bays and rivers, around rocky points and capes, between islands, and in limited or shallow water depth areas. The combination of these factors results in a complex dynamic that must be considered in locating and designing a tidal powered energy system.
River streams, especially those fed by rains and melting glaciers are also a plentiful and predictable energy resource. The magnitude of river stream energy depends upon the river's velocity and volume, the former determined by the gradient and the roughness and shape of the channel, and the latter determined by the size of the drainage basin, vegetation, climate, and permeability of the underlying rocks. The combination of these factors results in a complex dynamic that must be considered in locating and designing a river stream powered energy system.
Water has a much higher density than air (832 times), so a single tidal or river stream generator can provide significant power at low tidal or river flow velocities as compared to wind currents. Moreover, because of the smaller value of kinematic viscosity of water (almost 1:20), compared to air, small size blades (100-200 mm chord) that operate at high Reynolds numbers result in a significantly improved aero(hydro)dynamic performance in water as compared to air. A wind machine designed to operate at similar Reynolds numbers would require blades 10 times bigger in chord in order to be effective at low speeds.
Water turbine designs in the prior art are adopted from existing wind turbine technology and are naturally similar to the designs of wind turbines: propeller-type horizontal axis wind turbine (“HAWT”), or vertical axis wind turbines (“VAWT”), including Darrieus-type and helical bladed Gorlov type turbines. Conventional energy conversion devices for both tidal and wind resources have well known aerodynamic (or hydrodynamic) energy extraction limits based on a classical 1-D model, the so-called “Betz Limit” of 16/27 (59.3%), and have their peak performance, indicated by the value of coefficient of power Cp, at relatively high values of tip speed ratio (5-8). As such, the variation in rotor speeds will be very large and the cut-out speeds low, as limited by absolute tip speeds.
In known hydrokinetic turbine designs, the blades rotate around a fixed vertical (VAWT) or horizontal (HAWT) axis. A hub comprises symmetrical blades (for bi-directional tidal turbines) affixed thereto, which accepts the current from both sides. For a given tidal current velocity, there is a rotational speed delivering the maximum power and a free wheeling rotational speed.
Some of the problems associated with the traditional fixed axis turbine designs in water are that the mechanically complex hub is submersed and requires a minimum water depth for operation. Additionally, the difference in the relative speed of the rotor blade at the hub and tip reduces the extraction efficiency in HAWT turbines. In water, this problem is compounded by variable underwater current velocities—faster near the water's surface and slowing significantly approaching the bottom. Reversible turbines with large diameter rotors result in low-rotational-speed problems and have comparatively lower performance, higher cost, and are more complicated than non-reversible turbines. The size of these turbines can be varied only by varying the rotor diameter (as the number of blades required is 2 or 3 regardless of the size) these designs cannot be efficiently adapted to sites with varying widths and depths.
The tidal or river stream turbine design must also take into account the difficulties of a submarine environment, including long term submersion in water and strong tides or current, which are steadier and more consistent than wind forces. Additionally, the local geography under which installation, operation and maintenance are to be performed, should be considered. Machinery submerged under water must be designed to function under water, and requires proper construction, insulation, and sealing from the corrosive effects of the ocean or river water. Further, the vibration of the support structure, under the influence of tides, waves, and streams and resulting mechanical stress, wear, and tear also need to be considered. Finally, the design must be economically viable.
A water current velocity of 3 m/s is caused by a level difference of less than half a meter. The conventional arrangement of hydroelectric turbines, where the entire water stream has to be ducted into and out of the hydro turbine is not a practicable solution for an energy source with a very small head and a very large flow.
Indian Patent Application Nos.: 910/MUM/2006, 1106/MUM/2006 and 1563/MUM/2007, PCT IN2008/00878 and U.S. Pat. No. 7,709,971, relate to a novel Linear Wind Powered Electric Generator suitable for ultra low speed class 2 wind sites. These basic concepts and principles can be applied for a hydrokinetic, water powered electric generator, but must take into account current velocity, which is about 3-4 times lower, kinematic viscosity, which is 20 times lower, and density which is 800-1100 times higher than wind. Moreover, free surface, natural gradients, variable speeds due to channel depth, and current variations caused by channel cross section need to be addressed in adapting the aforementioned wind turbines for use in water flows.